At this point let it be noted that the discussion here is in fact of a cable length sensor and accordingly of a cable drum and a measuring cable but, however, the term “measuring cable” is to be understood in the sense of “means for measurement”. When using a measuring cable it can be of any material, e.g. of metal or metal alloys, of single-conductor metal, or of textile of the most varied type of manufacture. Instead of using a measuring cable, using a measuring band is possible, where said band can also be made of textile material, of plastic, or of metal. Finally, it is essential that the medium serving for the measurement is made so as to be stable longitudinally or with respect to drawing. What the means are in a concrete case is not essential for the teaching claimed.
Cable length sensors of the generic type have been known for years in the most varied embodiments. Among the essential components of such a cable length sensor is the measuring cable which is fastened on its end side to the object to be detected in its current position. The measuring cable is wound on the cable drum and is pre-tensioned by the return element. Depending on the application in question, a respective sensor can be provided, which is coupled to the cable drum. Since the diameter of the cable drum is known, the cable length drawn off can be determined via the sensor via an evaluation unit, which is usually external.
In the known cable length sensor the measuring cable is wound onto the cable drum, where the return element is pre-tensioned at least slightly in the completely wound-on state of the cable drum. If the object and the sensor move relative to one another, the measuring cable is drawn from the cable drum. In the reverse direction of motion, therefore when the measuring cable is once again wound onto the cable drum due to the return force of the return element, a motion of the object towards the cable drum or towards the sensor element occurs.
As published state of the art let us name, merely by way of example, EP 0 778 239 B1, which shows a generic cable length sensor. Furthermore, let us refer to U.S. Pat. No. 443,888, which also shows a generic cable length sensor. In both cases the cable drum and the return device are disposed in a housing, where, for replacing the return device, it is necessary to replace the entire cable length sensor and to dismount it—at the factory.
In regard to the state of the art known from practice let it be pointed out that the cable length sensors of the generic type frequently use wire potentiometers or hybrid potentiometers as sensor elements. A significant disadvantage in the case of these sensor elements is their short service lifetime, which in this respect defines the service lifetime of the entire cable length sensor. For this reason, conductive plastic potentiometers or wear-free inductive, magnetic, or optical sensors, among others, have been used in the meantime, since their service lifetime is significantly longer. In such a case the service lifetime of the cable length sensor is not limited by its respective sensor element but rather routinely by the service lifetime of its return device. Typical service lifetimes for commercially available return devices are in the range from 1 to 2 million cycles, where one cycle corresponds to one complete winding off and one complete winding on of the measuring cable. With unfavorable conditions of use, e.g. at high cable speeds and/or accelerations of the measuring cable, the service lifetime of the cable length sensor can be clearly reduced.
If the cable length sensor is used with very frequent motion, an extremely low short service lifetime is to be expected, since, specifically, damage occurs very frequently, even after a few months. This situation is customarily countered either by a damage-related replacement of the sensor or by a preventive replacement of the sensor in the framework of a relatively short service interval. Instead of replacing the entire cable length sensor, it is also conceivable to replace merely the spring elements of the return device, which, however, entails a significant effort.
It is to be noted that a break in a spring in the return device routinely leads to additional damage, e.g. to a tear in the measuring cable. In such a case it is necessary to dismount the complete cable length sensor and to mount a replacement device. Dismounting and mounting on site by unqualified personnel is practically not possible so that a special repair service is necessary. Among the significant disadvantages are, in particular, also long machine downtimes, since specifically the complete cable length sensor must be removed and a new cable length sensor must be installed, where, due to storing a complete cable length sensor in advance, high storage costs arise.
Furthermore, it is necessary in the case of a complete replacement of the cable length sensor to calibrate the control and/or the entire cable length sensor. The defective cable length sensor is customarily sent to the manufacturer, which entails a significant administrative overhead as well as transport costs. Since the conventional repair of the cable length sensor is very labor-intensive, relatively high repair costs arise. Additional costs are to be expected in case of consequential damage to the cable length sensor.
If one wishes to prevent an instance of damage, preventive replacement within a service interval presents itself in the case of conventional cable length sensors. In this case the sensor is to be replaced, more or less regularly, or the cable length sensor in use is to be removed each time. A new cable length sensor, always to be stored in advance, is to be installed by skilled personnel, which in accordance with the discussions above, entails significant storage costs for storing a cable length sensor in advance.
After the installation of the new cable length sensor, the control and/or the complete cable length sensor must be recalibrated. The replaced cable length sensor is sent to the manufacturer, due to which, however, costs arise, namely, on the one hand, costs due to administration and, on the other hand, costs due to transport. The cable length sensor sent to the manufacturer is generally overhauled, which, once again, is labor-intensive.
If one takes into consideration the fact that the return device's springs or spring packs previously indicated as a weak point are replaced separately, then this could theoretically be done on site or at the manufacturer's. In the case of prior-art cable length sensors, thus, for example, according to EP 0 778 239 B1 or U.S. Pat. No. 443,888, the spring replacement alone represents an extremely labor-intensive process. Furthermore, highly qualified technical personnel are necessary, which, in particular, have at their disposal the special tools needed in each case. Moreover, for numerous forms of construction there is a quite significant danger of injury due to a spring possibly jumping out of the housing as soon as the housing is opened and the spring of the return device are withdrawn.
The work to be carried out in replacing the spring of the return device is particularly complex, in particular when the cable length sensor has several springs disposed in series or in parallel. To replace the springs the complete housing, or at least the spring box, must be opened. The springs must be de-tensioned, where a pre-tensioning of typical spring packs can be up to 30 revolutions. If the springs, after the de-tensioning, are replaced, care must be taken that they are connected to one another and/or to the turning axle. If new springs or spring packs are introduced, they must be compressed to generate tension. This is probably the most laborious step of the operation since up to 150 spring revolutions are necessary for this. It is not possible to compress the springs without a special tool. Moreover, there is the danger that in compressing the springs, errors are made in the permissible number of revolutions, which in turn reduces the service lifetime of the spring or the spring pack. Furthermore, it is disadvantageous that the work necessary to replace the individual springs is, as a rule, only possible when the complete cable length sensor has been dismounted from the respective system. This also requires a quite significant investment of time and ultimately causes downtime of the respective machine, system, and so on.
In light of the explanations above, the present invention is based on the objective of developing and extending the cable length sensor of the generic type so that maintenance and repair of the complete cable length sensor is possible with as little effort as possible, even by less qualified personnel.